In recent years, to provide crash safety of automobiles for the occupants and to improve the fuel efficiency of automobiles by decreasing the weight of automobile bodies, the application of high-strength steel sheets having a TS of 780 MPa or more and a small thickness to structural members of automobiles has been promoted. In particular, the application of steel sheets having a very high strength of 980-MPa-grade TS or 1180-MPa-grade TS has also been recently studied.
However, in general, an increase in the strength of steel sheets results in degradation of the ductility, stretch flangeability, and bendability of steel sheets and leads to degradation of the formability of steel sheets. Accordingly, galvanized steel sheets having a high strength, excellent formability, and excellent corrosion resistance are demanded.
To meet such a demand, for example, Patent Literature 1 proposes a high-strength galvannealed steel sheet having excellent formability and excellent adhesion of the coating. This steel sheet contains, on the basis of mass percent, 0.04 to 0.1% C, 0.4 to 2.0% Si, 1.5 to 3.0% Mn, 0.0005 to 0.005% B, 0.1% or less P, more than 4N and 0.05% or less Ti, and 0.1% or less Nb, the balance being Fe and incidental impurities; has galvannealing coatings on the surfaces of the steel sheet, the galvannealing coatings containing 5 to 25% Fe; has a multi-phase microstructure of a ferritic phase and a martensitic phase; and has a TS of 800 MPa or more. Patent Literature 2 proposes a high-strength galvannealed steel sheet having good formability that contains, on the basis of mass percent, 0.05 to 0.15% C, 0.3 to 1.5% Si, 1.5 to 2.8% Mn, 0.03% or less P, 0.02% or less S, 0.005 to 0.5% Al, and 0.0060% or less N, the balance being Fe and incidental impurities; satisfies (Mn %)/(C %)≧15 and (Si %)/(C %)≧4; and has a volume fraction of 3 to 20% in terms of a martensitic phase and a retained austenitic phase in a ferritic phase. Patent Literature 3 proposes a high-strength galvanized steel sheet with low yield ratio that has excellent stretch flangeability. This steel sheet contains, on the basis of mass percent, 0.04 to 0.14% C, 0.4 to 2.2% Si, 1.2 to 2.4% Mn, 0.02% or less P, 0.01% or less S, 0.002 to 0.5% Al, 0.005 to 0.1% Ti, and 0.006% or less N, satisfies (Ti %)/(S %)≧5, the balance being Fe and incidental impurities; has a volume fraction of 6% or more in terms of the total of a martensitic phase and a retained austenitic phase; and satisfies α≦50000×{(Ti %)/48+(Nb %)/93+(Mo %)/96+(V %)/51} where α% represents the volume fraction of a hard phase microstructure including the martensitic phase, the retained austenitic phase, and a bainitic phase. Patent Literature 4 proposes a high-strength galvanized steel sheet having excellent adhesion of the coating and excellent elongation during forming of the steel sheet. This steel sheet contains, on the basis of mass percent, 0.001 to 0.3% C, 0.01 to 2.5% Si, 0.01 to 3% Mn, and 0.001 to 4% Al, the balance being Fe and incidental impurities; has galvanization coatings on the surfaces of the steel sheet, the galvanization coatings containing, on the basis of mass percent, 0.001 to 0.5% Al and 0.001 to 2% Mn, the balance being Zn and incidental impurities; satisfies 0≦3−(X+Y/10+Z/3)−12.5×(A−B) where, on the basis of mass percent, X represents the content of Si of the steel, Y represents the content of Mn of the steel, Z represents the content of Al of the steel, A represents the content of Al of the galvanization coatings, and B represents the content of Mn of the galvanization coatings; and has a microstructure that is constituted by a primary ferritic phase (volume fraction: 70 to 97%) having an average grain size of 20 μm or less and a secondary phase (volume fraction: 3 to 30%) constituted by a austenitic phase and/or a martensitic phase, the secondary phase having an average grain size of 10 μm or less.